This invention relates in general to location determining apparatus using satellite signals, and more particularly to satellite positioning receivers which have frequency errors introduced during manufacture of the receiver, and where such errors change over a period of time after manufacturing the receiver.
Satellite positioning receivers have been employed for a number of years, and are now implemented in integrated circuit form, making them both relatively small and inexpensive compared to their size and cost only a few years ago. Consequently, satellite positioning receivers are being used in many more applications than they have been in the past. For example, it is now relatively common to find them in automobiles for use with mapping and navigation equipment.
Presently, in their most common form, satellite positioning receivers are provided in integrated circuit form, and require the addition of some periphery circuits, such as reference oscillators. Of course, reference oscillators can be provided with oscillator circuits that are stable and precise. In general, thermal stability and precision are a function of cost. That is, the more stable and precise the oscillator circuit, the more it will cost, in general. However, in cost sensitive consumer electronics, it is desirable to use inexpensive circuitry.
Among the less expensive means of implementing oscillators, crystal oscillators are relatively stable and precise for their cost. Although other inexpensive and unsophisticated oscillator means exit, crystal oscillators are one of the most widely used circuits in electronic equipment. For most consumer electronics, crystal oscillators are stable and precise enough, and typically do not require correction. However, it is typical for crystal oscillators, as well as other types of oscillators, to be specified with a frequency precision measured in parts per million. For example, a crystal oscillator with a nominal operating frequency of 10 megahertz (MHz), with an error of xc2x15 parts per million (ppm) will have a frequency error of xc2x150 Hz. Although small in comparison to the nominal frequency, it is an unacceptable error in frequency sensitive applications, such as communications applications where channels are specified by frequency. A 50 Hz error could easily cause a communication signal to drift into a channel adjacent to the intended channel.
There are a number of techniques used to correct frequency errors. These techniques are not exclusive, so their corrective effect may be aggregated to establish a precise reference oscillator. For example, more precise oscillators are available, so instead of using a 5.0 ppm oscillator device, the designer may choose to use a lower tolerance part having a precision of 0.5 ppm. A common technique is to use automatic frequency correction (AFC). AFC can be performed by a variety of techniques. One of the more common techniques in communication device is where the device receives a precision carrier signal and compares its internal oscillator frequency to the received precision carrier to determine a frequency error or offset. The offset is used to correct the reference oscillator frequency by, for example, a frequency synthesizer or other frequency dependent operators within the device.
Despite the use of more precise oscillator devices, it has been found that, as a result of manufacturing processes, the frequency of an oscillator device can change more than its specified tolerance. For example, the 0.5 ppm can apply over temperature over a short period of time, but the crystal can experience larger changes over greater periods of time due to aging or mechanical shock, or both. Specifically, when, for example, a crystal oscillator device is exposed to intense heat, such as during circuit board solder reflow, a crystal having a 0.5 ppm specified tolerance can change by as much as ten times that, or 5.0 ppm. Furthermore, for a period of time after exposure to high heat, the crystal frequency will continue to change back towards its nominal frequency. The period of time typically lasts about 2 days, and then the crystal frequency will settle, and thereafter the changes in frequency will be relatively small, assuming the device is not exposed to any high temperatures. This change in frequency causes a problem because after manufacture of the device, the device is typically tested and tuned. If the device is tested and tuned at a time shortly after manufacture, the tuning will be ineffective because by the time a user is ready to use the device, the operating frequency of the oscillator will have changed from what it was shortly after manufacture. It is typically expected that the frequency will change, and in devices that receive signals, there is a range of frequency the device will search to acquire a carrier signal, and then the device will correct its frequency offset, if necessary. However, it has been found that in some instances, the frequency may change so much between the time the device is initially tested and the first time it is used by an end user that the search window may not be broad enough to locate a desired carrier signal. A broader search window could be used, but a broader search window increases the amount of time a device may search for a desired carrier signal. Therefore there is a need for a means by which oscillator frequency change subsequent to manufacture of the device is accommodated without impacting the search time during regular operation.